1. Field of the Invention:
The present invention relates to cationic electrodeposition compositions and to their use in the method of electrodeposition. More particularly, this invention relates to cationic electrodeposition compositions which are capable of being electrodeposited as thick films.
2. Brief Description of the Prior Art:
Electrodeposition as a coating application method involves deposition of a film-forming composition under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers higher paint utilization, outstanding corrosion protection and low environmental contamination. Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
One disadvantage associated with many of the cationic electrodeposition coatings is that they electrodeposit as very thin films, that is, on the order of 10 to 20 microns. This shortcoming can be overcome by using relatively large quantities of organic solvent such as glycol monoalkyl ethers and alcohols but this is undesirable because the organic solvents often evaporate from the large open electrodeposition tanks causing their level and their effect on film build to vary. Also, these solvents are lost on baking and pose an air pollution problem.
It is also known in the art to incorporate non-volatile nonreactive components in the electrodeposition bath to effect film build. However, many of these materials must be used at high levels and are undesirable because they adversely affect the rupture voltage of the film during electrodeposition. Also, the properties of the resultant coating such as corrosion resistance, particularly coatings which are undercured, are adversely affected. Also, these materials are often difficult to incorporate into the bath.
The ideal additive would provide a large increase in film build with good appearance. Also, properties such as rupture voltage and corrosion resistance would not be adversely affected. The additive should be non-volatile, easy to incorporate into the bath and also have a limited water solubility such that it will not build up in the bath as the electrodeposition proceeds.
A group of materials that are especially effective in this regard are hydroxyl-free, alkylated polyethers containing at least three ether oxygens per molecule and from 1 to 4 saturated hydrocarbon groups separating the ether groups. The alkyl groups contain on an average of at least 2 carbon atoms. The polyethers are non-volatile and have a limited solubility in water.